This invention relates to a method for manufacturing an electrically conductive electrode for use in batteries. The invention also relates to the application of said electrode as the cathode in a primary alkaline battery.
Primary alkaline electrochemical cells typically include a zinc anode active material, an alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulosic and synthetic fibers. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector. The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. The cathode typically includes manganese dioxide as the electrochemically active material admixed with an electrically conductive additive to enhance electrical conductivity, an optional polymeric binder, and other optional additives, such as titanium-containing compounds including anatase-type titanium dioxide and other alkaline earth metal titanates. Because manganese dioxide typically exhibits relatively low electrical conductivity, an electrically conductive additive is needed to improve the electrical conductivity between individual manganese dioxide particles and also between the manganese dioxide particles and the steel container that encloses the cell components and that also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, natural graphites, synthetic graphites, including expanded or exfoliated graphites, and combinations thereof.
It is desirable for a primary alkaline battery to have a high discharge capacity (i.e., long service life). Since commercial cell sizes have been fixed, it is known that the performance and/or useful service life of a cell can be enhanced by increasing the interfacial surface area of the electrode active materials as well as by packing greater amounts of the electrode active materials into the cell. However, these approaches have practical limitations such as, for example, if the electrode active material is packed too densely in the cell, the rates of electrochemical reactions during cell discharge can be reduced, in turn reducing service life. Other deleterious effects such as cell polarization can occur as well. Polarization limits the mobility of ions within both the electrolyte and the electrodes, which in turn degrades cell performance and service life. Although the amount of active material included in the cathode typically can be increased by decreasing the amount of non-electrochemically active materials such as polymeric binder or conductive additive, a sufficient quantity of conductive additive must be maintained to ensure an adequate level of bulk conductivity in the cathode. Thus, the total active cathode material is effectively limited by the amount of conductive additive required to provide an adequate level of conductivity.
Further, it is highly desirable to enhance the performance of an alkaline cell at high rates of discharge. Typically, this is accomplished by increasing the volume fraction of conductive additive in the cathode in order to increase overall (viz., bulk) electrical conductivity of the cathode. The volume fraction of conductive additive within the cathode must be sufficient to form a suitable percolative network of conductive particles. Typically, when the conductive additive is a conductive carbon, about 5 to 15 weight percent of the total mixture is required. However, an increase in the amount of conductive carbon produces a corresponding decrease in the amount of active cathode material, giving lower service life. Conventional powdery conductive carbons such as acetylene black and flaky, crystalline natural or synthetic graphites have intrinsic drawbacks including low packing density, high electrolyte absorption, and high levels of impurities that can lead to excessive hydrogen gassing by the cell.
It is well known to use a specific type of graphite called expanded graphite in place of conventional powdery conductive carbons in battery cathodes. As used herein, expanded graphite comprises natural or synthetic graphite in which the crystal lattice has been uniaxially expanded or exfoliated. Various methods can be employed to form expanded graphite including, for example, the incorporation of a strong acid such as sulfuric, nitric, or chromic acid or mixtures thereof and a strong oxidant such as hydrogen peroxoide, perchloric acid, iodic or periodic acid, perchloric acid salts, permanganate salts, and the like followed by a rapid high temperature treatment as disclosed, for example, in U.S. Pat. Nos. 1,137,373; 1,191,383; 3,404,061; and Japanese Unexamined Patent Application (Kokai) No. 16406/1994. Following the heat-treatment, the expanded graphite typically is washed, compacted, and milled by attrition to produce the desired average particle sizes. After milling, expanded graphite particles typically exhibit reduced thickness in the direction of the graphite crystallographic c-axis. Since decreased particle thickness results in an increase in the number of conductive graphite particles per unit weight, a specific weight fraction of expanded graphite can impart a higher degree of conductivity in a cathode than the same amount of a non-expanded graphite. When admixed with manganese dioxide to form a cathode, less expanded graphite can be used resulting in increased service life. In addition, as disclosed in U.S. Pat. No. 5,482,798, expanded graphite has a flaky particle morphology, high compressibility, high lubricity, and good moldability thereby facilitating cathode fabrication.
The use of expanded graphite as a conductive additive in cathodes of conventional alkaline primary cells is known and disclosed for example, in U.S. Pat. No. 5,482,798; PCT publication no. WO 93/08123; European Application EP 0170,411; and also in Japanese Unexamined Patent Applications (Kokai) JP56-128579 and JP56-118267. A suitable expanded graphite having an average particle size ranging from 0.5 to 15 microns, preferably from 2 to 6 microns is disclosed in the ""798 patent. The smaller average particle size of expanded graphite relative to conventional natural or synthetic crystalline graphite (e.g., 15 to 30 microns) was hypothesized to facilitate the formation of a conductive network typically at a lower volume fraction of graphite. An expanded graphite having an average particle size greater than about 30 microns was disclosed to provide no performance advantage in alkaline cells compared to a conventional non-expanded natural graphite having a comparable particle size. The ""798 patent also disclosed that suitable amounts of expanded graphite can range from about 2 to 8 weight percent, and preferably from about 3 to 6 weight percent of the total cathode. Further, for expanded graphite contents of greater than about 10 weight percent no performance advantage is provided relative to equivalent amounts of unexpanded graphite particles in alkaline primary cells.
Various methods for preparing mixtures of manganese dioxide and graphite are known to provide suitable levels of electrical conductivity in cathodes of alkaline cells. Typically, graphite can be mixed dry with manganese dioxide using any of a variety of conventional blending, mixing or milling equipment. For example, U.S. Pat. No. 5,938,798 discloses the use of a twin cylinder mixer or a rotary tumbling mixer to dry mix graphite and manganese dioxide. In a subsequent step disclosed in the cited ""798 patent, the formed mixture was wet-pulverized, preferably in water, using a horizontal media mill such as a ball mill or bead mill to decrease average manganese dioxide particle size to less than 10 microns. Excessive pulverization of either the mixture of graphite and manganese dioxide or manganese dioxide in the absence of graphite was disclosed to result in degraded cell discharge performance. It was further disclosed that graphite can function as a lubricant during the pulverization process thereby permitting reduction of manganese dioxide particle size without degradation of electrochemical properties.
Manganese dioxides suitable for use in alkaline cells include both chemically produced manganese dioxide known as xe2x80x9cchemical manganese dioxidexe2x80x9d commonly referenced in the art as xe2x80x9cCMDxe2x80x9d and electrochemically produced manganese dioxide known as xe2x80x9celectrolytic manganese dioxidexe2x80x9d commonly referenced as xe2x80x9cEMDxe2x80x9d. CMD can be produced economically and in high purity, for example, by the methods described by Welsh et al. in U.S. Pat. No. 2,956,860. EMD is manufactured commercially by the direct electrolysis of a bath containing manganese sulfate dissolved in a sulfuric acid solution. Manganese dioxide produced by electrodeposition typically has high purity and high density. Processes for the manufacture of EMD and representative properties thereof are described in xe2x80x9cBatteriesxe2x80x9d, edited by Karl V. Kordesch, Marcel Dekker, Inc., New York, Vol. 1, 1974, pp.433-488. EMD is the preferred type of manganese dioxide for use in alkaline cells. One consequence of the electrodeposition process is that the EMD typically retains a high level of surface acidity from the sulfuric acid of the electrolysis bath. This residual surface acidity can be neutralized for example, by treatment with an aqueous base solution. Suitable aqueous bases include: sodium hydroxide, ammonium hydroxide (i.e., aqueous ammonia), calcium hydroxide, magnesium hydroxide, potassium hydroxide, lithium hydroxide, and any combinations thereof. Typically, commercial EMD is neutralized with a strong base such as sodium hydroxide because it is both highly effective and economical.
Thus, even though considerable effort has been expended, as evidenced by the prior art cited hereinabove, the methods used to prepare cathodes containing EMD and graphite require additional refinement in order to improve substantially the discharge performance and service life of alkaline electrochemical cells.
It is a principal objective of the present invention to produce an improved cathode comprising manganese dioxide and an electrically conductive carbon that provides an improvement in discharge performance and service life of alkaline electrochemical cells when included therein.
An aspect of the present invention is directed to producing an alkaline primary cell having increased service life as well as improved discharge performance at high rate of discharge by increasing the volume fraction of electrochemically active material within the cathode. Because the overall dimensions of commercial alkaline cells are fixed, the internal volume available for the component materials is also fixed. In order to increase the total amount of active cathode material, the volume fraction of electrochemically-inactive components must be decreased. The present invention provides for decreasing the amounts of non-electrochemically active components in the cathode, including for example polymeric binders and conductive additives, while preserving the overall level of electrical conductivity and mechanical integrity of the cathode.
The present invention also provides a conductive cathode consisting of predominantly manganese dioxide admixed with a small amount of an electrically conductive carbon wherein the carbon functions both as a conductive additive and a binder. The conductive carbon can be selected from acetylene black, natural or synthetic flaky, crystalline graphite, expanded graphite prepared from a natural or synthetic graphite precursor, graphitized carbon fibers, including carbon nanofibers, nanotubules or fibrils and combinations thereof.
In an aspect of the process of the invention, cathodes suitable for alkaline electrochemical cells can be formed by mechanically agitating an essentially dry admixture of particulate manganese dioxide and conductive carbon with inert, rigid milling media, separating the milling media from the mixture, and optionally adding aqueous electrolyte to the mixture to facilitate cathode fabrication. Suitable conductive carbon particles can include, for example, acetylene blacks, natural crystalline or synthetic crystalline flaky graphites, expanded graphites (which can also be in exfoliated form), graphitized carbon fibers, including carbon nanofibers, nanotubules or fibrils or any mixture thereof.
Another aspect of the present invention provides a high-efficiency mixing or blending process whereby manganese dioxide and conductive carbon particles can be sufficiently intermixed such that the volume fraction of conductive carbon can be minimized and the volume fraction of manganese dioxide can be maximized in the cathode while maintaining a suitable level of electrical conductivity. At low volume fractions of conductive carbon, the conductivity of the cathode depends strongly on the efficiency of percolative network formation by the conductive carbon particles, which in turn depends on the average particle sizes and particle size distributions, as well as the relative sizes of the manganese dioxide and conductive carbon particles, the particle morphologies, the specific conductivity of the conductive carbon particles, and especially, the homogeneity of the intermixing of the conductive carbon and manganese dioxide particles.
Still another aspect of the present invention includes the mixing process whereby manganese dioxide and conductive carbon particles can be sufficiently intermixed in the dry state, that is, without the addition of liquids or solvents to form slurries or dispersions thereof. The mechanical mixing process of the invention provides a simplification of the typical cathode manufacturing process whereby the final steps of separating the mixture of manganese dioxide and conductive carbon from an added liquid or solvent and drying of the mixture can be eliminated. Further, the dry mixing process of the invention advantageously minimizes degradation of cell discharge performance resulting from attrition of the manganese dioxide particles during mixing by conventional wet-milling processes.